Known water-soluble cellulose derivatives include hydroxypropyl cellulose which is used for binders and coating agents of tablets. Independently, of cellulose acetates, cellulose acetates with a low degree of substitution having a total degree of acetyl substitution of about 0.3 to about 1.2 are known to be soluble in, or have high affinity for, water.
For example, Japanese Examined Patent Application Publication No. H01-13481 discloses a water-soluble cellulose acetate having a total degree of acetyl substitution of 0.3 to 0.8 and a ratio of a degree of acetyl substitution at the 6-position to the total degree of acetyl substitution of from 0.35 to 0.4. This literature describes that the cellulose acetate is obtained by hydrolyzing a highly substituted cellulose acetate having a total degree of acetyl substitution of 2.4 or more in the presence of a large amount of water by the catalysis of an inorganic acid such as hydrochloric acid or sulfuric acid.
Japanese Unexamined Patent Application Publication No. H04-261401 discloses a water-soluble cellulose acetate having a total degree of acetyl substitution of 0.4 to 0.9, a water-insoluble matter content of 1.5 percent by weight or less, and a weight-average molecular weight of 5×103 to 3×106. This literature describes that the water-soluble cellulose acetate is obtained by the combination of an acetylation step, a first hydrolysis step, and a second hydrolysis step. In the acetylation step, cellulose is acetylated at a high temperature for a short time in the presence of a small amount of an acidic catalyst. In the first hydrolysis step, the resulting cellulose acetate having a high degree of acetyl substitution is hydrolyzed at a high temperature for a short time. In the second hydrolysis step, the cellulose acetate is further hydrolyzed at a relatively high temperature for a short time in the presence of a small amount of an acidic catalyst.
Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. H05-500684 and Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. H05-501129 disclose methods in which a cellulose ester is subjected to a treatment with a specific solvolysis promoter in the presence of a specific solvent to give a cellulose ester having a lower degree of substitution. The literature mentions that the methods give a cellulose ester having a degree of substitution of 0.4 to 1.2 from a cellulose ester having a degree of substitution of 2 to 3.